1. Technical Field
The present invention relates to a technology of an image pickup element capable of receiving an object light beam transmitted through a shooting optical system.
2. Background Art
In an image pickup device such as a single-reflex digital camera of a lens-replacement type, the use of an image pickup element (may hereunder be also referred to as a phase-difference detecting image pickup element) is proposed. The image pickup element includes a plurality of pairs of pixels (hereunder may also be referred to as “AF pixel pairs”) and is capable of performing focus detection by a phase difference detection method. Each AF pixel pair generates a corresponding pixel signal by receiving object light beams transmitted through a pair of portion areas (such as a left pupil portion/a right pupil portion) in an exit pupil of an interchangeable lens (shooting optical system).
In the phase-difference detecting image pickup element, at each AF pixel pair separated from the center thereof, the object light beams used in the focus detection may be limited due to vignetting of the shooting optical system. In such a case, an imbalance may occur in the pixel signals generated at the AF pixel pairs. As disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-191629, this problem can be mitigated by reducing the imbalance as a result of performing a corrective calculation on the pixel signals generated at the AF pixel pairs on the basis of a ratio between a pupil width and a shift amount with respect to an optical axis of the pupil limited by the vignetting.
However, in the technology discussed in the aforementioned Japanese Unexamined Patent Application Publication No. 2004-191629, if the imbalance in the pixel signals generated at the AF pixel pairs becomes excessive, the imbalance cannot be completely corrected even if the corrective calculation is performed, thereby preventing the imbalance from being properly corrected. In particular, even when, for example, an interchangeable lens in which the position of an exit pupil with respect to the image pickup element is considerably different from an assumed position is mounted, the imbalance in the pixel signals generated at the AF pixel pairs is excessive. Even in such a case, it is difficult to eliminate the imbalance by performing the corrective calculation described in the aforementioned Japanese Unexamined Patent Application Publication No. 2004-191629, and to precisely detect the focus.
The present invention is achieved in view of the aforementioned problems, and has as its object the provision of a technology of a phase-difference detecting image pickup element that can precisely perform focus detection even if the position of an exit pupil with respect to the image pickup element changes.